1. Field of the Invention
The present invention relates generally to a method of transmitting DPCH (Dedicated Physical Channel) frames between an RNC (Radio Network Controller) and a UE (User Equipment) in a CDMA (Code Division Multiple Access) communication system, and in particular, to an ATTA (Adaptive Transmission Time Adjustment) method for adaptively adjusting transmission time between an RNC and a UE.
2. Description of the Related Art
In general, an RNC adjusts transmission time for data communication with a UE via a node B in a CDMA communication system. For the transmission time adjustment, the UE transmits an uplink DPCH frame T0 chips (1024 chips) after receiving a downlink DPCCH (Dedicated Physical Control Channel)/DPDCH (Dedicated Physical Data Channel). The DPCCH and the DPDCH are collectively called the DPCH. In real implementation, however, a propagation time delay is involved according to the distance between the node B and the UE. In this case, a serving RNC (SRNC) requests the UE to report the time difference between the received time of the downlink DPCH (DL DPCH) frame and the transmitted time of the uplink DPCH (UL DPCH) frame (hereinafter, referred to as UE Rx−Tx time difference). The SRNC adjusts a DPCH frame offset IE (Information Element) in an RRC (Radio Resource Control) Reconfiguration message based on the UE Rx−Tx time difference in order to inform the UE about the transmission time adjustment. The SRNC only transmits the transmission time adjustment information to the UE but cannot adjust the transmission time of the UE.
Specifically, an RNSAP (Radio Network Subsystem Application Part)/NBAP (Node B Application Part) message has no IE related with downlink transmission time adjustment and thus does not support the downlink DPCH transmission time adjustment of the SRNC.
The UE usually transmits an uplink DPCH frame To chips after receiving a downlink DPCH frame. Here, an initial adjustment error must be limited within ±1.5 chips. The UE then adjusts its transmission time within a ¼ chip duration every time period of 200 ms from the downlink DPCH frame.
When the UE enters a handover region, it receives downlink DPCH data from a plurality of cells. Since each DPCH data has a different propagation delay, the UE cannot adjust its uplink DPCH transmission time. The UE performs a tracking process while implementing a handover from a source node B to a target node B. Also in the case, the UE receives signals with different propagation delays from the source node B and the target node B and thus cannot adjust its transmission time.
When the UE moves out of the handover region, the variance a of the time difference between the received time of the downlink DPCH frame and the transmitted time of the uplink DPCH frame may exceed an allowable value of 128 chips. Therefore the UE cannot adjust its transmission time.
FIG. 1 illustrates a time relationship between downlink DPCH data and uplink DPCH data in a handover region on the assumption that the UE moves from cell 1 to cell 2. The UE adjusts its transmission time according to the transmission time of cell 1, at every predetermined time during handover, and then according to the transmission time of cell 2, after the handover is complete.
According to the first method, the UE transmits an uplink DPCH 103 time T0 after receiving a downlink DPCH 101 from cell 1. In the second method, that is, when the UE enters the handover region, it receives downlink DPCHs 101, 105, 107, 110, 111 and 113 with different propagation delays from cell 1 and cell 2. Thus, the UE transmits uplink DPCHs 103, 109 and 112 in every predetermined time period without adjusting its transmission time. Then, when the UE moves out of the handover region and enters the coverage area of cell 2, the time difference of the received time of a downlink DPCH 115 and the transmitted time of an uplink DPCH 117 becomes T0+α. The UE performs a tracking process within a ¼ chip duration in the unit of 200 ms and thus the UE cannot adjust the transmission time of the uplink DPCH.
As described above, the conventional CDMA communication system has no path in which a control signal for robust transmission time adjustment between a downlink DPCH and an uplink DPCH is transmitted. Therefore, when the UE enters a handover region, it cannot adjust the transmission time of the uplink DPCH. Moreover, the tracking on the basis of the ¼ chip duration is insufficient to ensure the DPCH transmission time adjustment.